JPH10314892A - Continuous casting method of high Ti content steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a continuous casting method for a high Ti-containing steel which prevents surface defect and can roll without cleaning, by containing a specific ratio of Ti, making the spouting hole angle of an immersion nozzle the upward angle having a specific range and dipping the immersion nozzle into a specific range of depth. SOLUTION: Molten steel 6 is poured into a mold 3 toward short sides 5 of the mold as spouted flow 9 from the spouting holes 2 arranged at the lower part of the immersion nozzle 1 through a flow-out hole 16, and the solidified shell 10 is formed and drawn out downward to make a continuously case slab. Ar gas is blown into the flow-out stream 16 from the fine pores in the upper nozzle 12 to prevent the sticking of TiO2 , etc., to the inner wall of the immersion nozzle 1. The spouting holes 2 are symmetrically arranged in the immersion nozzle 1 and the spouting hole angle θ is set at 2-8 deg. upward direction (desirable at 4-6 deg.). The dipping depth H is set to 80-130 mm (desirable at 100-110 mm under consideration of the positional variation of a meniscus 7). By this method, the strong cast slab of the Ti-containing steel having 0.1-1.0 wt.% without entrapping slag and developing longitudinal crack, is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of high Ti content steel for optimizing the shape and depth of a dipping nozzle and stably producing a slab having few surface defects.

[0002]

2. Description of the Related Art T is a typical type of precipitation-type wear-resistant steel.
i is 0.1 to 1.0 wt% (hereinafter simply referred to as “%”)
In the high Ti content steel contained, Ti in molten steel is oxidized by inevitable oxidation during continuous casting, and TiO ₂ is generated. This TiO ₂ is used for the molten steel surface in the mold (hereinafter referred to as “meniscus”).
) Is concentrated in the mold powder added above to precipitate a high melting point compound (CaTiO ₃ ), thereby changing properties such as viscosity and melting point of the mold powder. For this reason, the amount of mold powder flowing into the space between the mold and the solidified shell (hereinafter referred to as “powder flowing amount”) is reduced to cause vertical cracks on the slab surface, So-called "norokami" is generated and quality is abnormal. In the worst case, seizure between the mold and the solidified shell occurs due to a shortage of the amount of powder flowing in, and a breakout operation accident occurs. Therefore, various measures have conventionally been taken to stably cast high Ti content steel.

For example, Japanese Patent Application Laid-Open No. 7-32091 (hereinafter referred to as “prior art 1”) discloses that a CaO-based flux is added to a ladle during or after tapping of a converter,
During continuous casting method for producing a high Ti containing steel is disclosed that CaO / SiO ₂ was used mold powder of 0.4 to 0.7. According to the prior art 1, the CaO-based flux absorbs TiO ₂ and Al ₂ O ₃ in the molten steel, thereby cleaning the molten steel and performing the CaO / mold powder molding.
By setting the SiO ₂ to 0.4 to 0.7, the formation of a high melting point compound in the mold powder can be prevented, so that clean high Ti-containing steel can be cast stably without operation accident. .

Japanese Patent Application Laid-Open No. 7-251251 (hereinafter referred to as "prior art 2") discloses that a mold is vibrated in a sine curve with a slab drawing speed in a range of 0.8 to 1.2 m / min. At this time, the mold vibration stroke S is set to 2 to 5 m.
m and the mold frequency f is 120 cycles / mi.
A method of continuously casting a high Ti content steel under vibration conditions that satisfy n or more and a mold frequency f × a mold vibration stroke S ≦ 600 is disclosed. According to Prior Art 2, since the vibration condition is set to a low stroke and a high frequency, even when using a mold powder generally used for casting ordinary steel, the oscillation mark can be made shallow, and the oscillating mark such as norokami can be reduced. It is said that surface defects can be prevented.

[0005]

The continuous cast slab is desirably supplied to the next hot rolling step without care in order to improve yield and save energy. However, in order to supply without care, it is necessary to prevent all surface defects such as vertical cracks and norokami.

[0006] The present inventors have studied a continuous casting method for maintaining a high Ti content steel without care, and as a result, it has been found that the above-mentioned prior art has the following problems. That is, in the prior art 1, although the formation of the high melting point compound in the mold powder is prevented, which is effective in preventing operation abnormalities such as breakout, depending on the casting conditions, vertical cracks or squeaks are generated, and stable. Can not be taken care of. In the prior art 2, the oscillation mark depth is shallow, but the mold vibration conditions are low stroke and high frequency, so that the amount of powder flowing in is reduced and vertical cracks may occur on the slab surface. , Can not be stable and careless.

As described above, it is difficult to say that the conventional method is sufficiently effective in preventing slab surface defects for maintaining high Ti content steel without care, and there is much room for improvement at present. It is.

[0008] The present invention has been made in view of the above circumstances,
An object of the present invention is to provide a continuous casting method of high Ti content steel which can prevent surface defects of a slab and can be rolled without care.

[0009]

According to the present invention, there is provided a continuous casting method for a high Ti content steel containing 0.1 to 1.0 wt% of Ti. The angle θ is in the range of 2 to 8 degrees upward, and the immersion depth H of the immersion nozzle is in the range of 80 to 130 mm.

From the results of the investigations by the present inventors, surface defects that hinder the maintenance of high Ti-containing steel are represented by norokami and vertical cracks, and the generation of these surface defects depends on the discharge flow from the immersion nozzle. It depends on the flow of meniscus due to molten steel.

In the case of high Ti content steel, the mold powder absorbs the TiO ₂ floating from the molten steel, so that the melting point of the mold powder rises. Mold powder having an increased melting point also has an increased viscosity. In order to prevent sloshing and prevent vertical cracks by sequentially absorbing the rising TiO ₂ , the mold powder whose melting point and viscosity have risen is melted uniformly, and also flows uniformly between the mold and the solidified shell. Need to be done. To this end, the molten steel in the meniscus must be constantly renewed, and heat, which is the heat of fusion of the mold powder, must be constantly supplied. Therefore, in continuous casting of high Ti content steel, it is necessary to supply more discharge flow from the immersion nozzle to the meniscus than in the case of ordinary carbon steel. However, if the flow rate of the molten steel in the meniscus is increased by increasing the supply amount too much, vortex or runaway occurs in the meniscus and unmelted mold powder is caught in the meniscus, resulting in mold powder looseness.

By properly controlling the discharge hole angle θ of the immersion nozzle and the immersion depth H of the immersion nozzle, the inventors can renew the molten steel without causing a vortex or runaway in the meniscus. It has been found that surface defects of high Ti content steel can be prevented. Note that the discharge hole angle θ of the immersion nozzle is defined as the angle between the direction of the discharge hole and the horizontal line, the direction of the discharge hole above the horizontal is upward, and the case below the horizontal is downward, The immersion depth H of the immersion nozzle is defined by the distance between the upper end position of the immersion nozzle discharge hole and the meniscus.

That is, if the discharge hole angle θ exceeds 8 degrees upward, the discharge flow flowing from the discharge holes toward the meniscus increases, and vortices, runaways, etc. occur in the meniscus, and the level of the molten metal becomes severe. Due to the fluctuation of the molten metal level, the unmelted mold powder is forcibly entangled in the molten steel, is captured by the solidified shell, and becomes stiff. Also, when the immersion depth H is increased and increased, the fluctuation of the surface of the meniscus is suppressed and the sloshing is reduced, but the renewal of molten steel is insufficient and the melting of the mold powder is hindered, and the amount of powder flowing in decreases. Longitudinal cracks increase. As described above, when the discharge hole angle θ exceeds 8 degrees upward, even if the immersion depth of the immersion nozzle is changed, it is impossible to simultaneously prevent the norokami and the vertical crack. The relationship between the state of occurrence of this surface defect and the immersion depth H is as follows:
FIG. 5 shows an example in which the ejection hole angle θ is 10 degrees upward (details will be described later). When the immersion depth is shallow, the fluctuation of the molten metal level becomes severe, the amount of powder flowing in becomes uneven in the width direction of the mold, and vertical cracks occur simultaneously.

If the discharge hole angle θ is less than 2 degrees upward, the discharge flow flowing in the horizontal direction increases, and the renewal of molten steel by the discharge flow in the meniscus decreases. As a result, the temperature of the molten steel of the meniscus is lowered, the melting of the mold powder is hindered, the amount of powder flowing in is reduced, and vertical cracks occur. Further, when the immersion depth H is made shallow to renew the molten steel in the meniscus, the vertical cracking is reduced, but the surface level of the meniscus fluctuates greatly and mold powder is involved and the slime increases. As described above, when the discharge hole angle θ is less than 2 degrees upward, even if the immersion depth H of the immersion nozzle is changed, it is not possible to simultaneously prevent the norokami and the vertical crack. The relationship between the state of occurrence of this surface defect and the immersion depth H is shown in FIG.
(Details will be described later).

When the discharge hole angle θ is in the range of 2 to 8 degrees upward and the immersion depth H is in the range of 80 to 130 mm, there is no fluctuation in the molten metal level in the meniscus and the renewal of the molten steel is performed smoothly. Norokami and vertical cracking can be prevented at the same time. However, if the immersion depth H of the immersion nozzle is less than 80 mm, the immersion depth H is too shallow, so that the meniscus causes a change in the molten metal surface and sloshing occurs. , Immersion depth H
Is too deep, renewal of molten steel in the meniscus is hindered, and vertical cracks occur. When the meniscus surface level fluctuates greatly, the amount of powder flowing in becomes uneven and vertical cracks occur simultaneously. Also, if the renewal of molten steel is hindered, the melting of the mold powder becomes insufficient and TiO ₂ The absorption capacity is reduced, and the rising TiO ₂ is trapped by the solidified shell, and slime is also generated. FIG. 3 shows the relationship between the state of occurrence of this surface defect and the immersion depth H, taking as an example the case where the discharge hole angle θ is 5 degrees upward (details will be described later).

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1 is a schematic view of a front section of a mold portion of a continuous casting machine having a rectangular cast slab to which the present invention is applied.

In FIG. 1, a tundish 11 is arranged above a mold 3 composed of opposed mold long sides 4 and opposed mold short sides 5 provided inside the mold long sides 4. At the bottom of the tundish 11 is an upper nozzle 12
A sliding nozzle comprising a fixed plate 13 and a sliding plate 14 is disposed on the lower surface side of the upper nozzle 12, and a rectifying nozzle 15 on the lower surface side of the sliding disk 14.
And the immersion nozzle 1 are arranged in order, and the tundish 1
An outflow hole 16 of molten steel from 1 to the mold 3 is formed. Molten steel 6 injected into tundish 11 from ladle (not shown)
Is provided at the lower part of the immersion nozzle 1 through the outflow hole 16, and the discharge flow 9 is directed into the mold short side 5 from the discharge hole 2 immersed in the molten steel 6 in the mold 3 into the mold 3. Injected. Then, the molten steel 6 is cooled in the mold 3 to form a solidified shell 10, and is continuously drawn below the mold 3 to be a cast piece. In FIG. 1, the discharge flow 9 is shown on only one side.

An Ar gas introducing pipe (not shown) is connected to the upper nozzle 12, and A gas is introduced through fine pores in the upper nozzle 12.
r gas, TiO ₂ and Al ₂ O on the inner wall of the immersion nozzle 1
₃ is blown into the outflow hole 16 to prevent adhesion.
On the meniscus 7 in the mold 3, a mold powder 8 is added.

The immersion nozzle 1 has a circular or elliptical outer shape, and a concave pool is formed at the bottom of the inner surface of the immersion nozzle 1. One discharge hole 2 is provided on each of the right and left sides with the center of the immersion nozzle 1 symmetrical, and the cross-sectional shape of the discharge hole 2 may be circular or elliptical. And immersion nozzle 1
Is set to an arbitrary value within a range from 2 degrees upward to 8 degrees upward. However, from the viewpoint of the stability of the slab surface properties, the discharge hole angle θ of the immersion nozzle 1 is preferably set to about 5 degrees upward (4 to 6 degrees) which is the median of the limited range.

Then, the immersion depth H is increased from 80 mm to 130
Casting is performed with an arbitrary value set in the range of mm. still,
The immersion depth H is limited to a boundary value of a limited range of 80 mm or 130 mm.
mm, the position of the meniscus 7 changes due to disturbance during casting, and the immersion depth H may be out of the limited range. Therefore, the immersion depth H is also set to the median side of the limited range of 100 mm to 110 mm. It is preferable to stably prevent surface defects.

The slab drawing speed is 0.7 to 1.8 m / mi.
n and a slab-shaped slab having a slab thickness of 150 to 250 mm and a slab width of 650 to 2300 mm. The thickness of the immersion nozzle 1 may be 10 mm to 50 mm, and the sectional area of the discharge hole 2 may be ²⁰ to 120 cm ² .

The mold powder 8 may be either for high Ti content steel or ordinary carbon steel. However, in order to stably prevent surface defects, mold powder for high Ti content steel is used. Is preferred.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention using a continuous casting machine having the structure shown in FIG. 1 will be described below.

The cross section of the slab has a thickness of 250 mm and a width of 150
In a continuous slab casting machine of 0 mm, the Ti content is 0.1 mm.
4% high Ti content steel with slab drawing speed 0.8m / min
Cast in. The used immersion nozzle has an inner diameter of 80 mm,
It has a wall thickness of 30 mm, a discharge hole cross-sectional area of 50 cm ² , and a concave pool of 15 mm in depth at the bottom. The discharge hole angle θ has three levels: upward 8 degrees, upward 5 degrees, and upward 2 degrees. And Then, the immersion depth H is set to 100 mm and 12
Ar gas was blown into a 9 Nl / min immersion nozzle using mold powder for high Ti content steel at two levels of 5 mm. In addition, as a comparative example, the immersion depth H is out of the limited range of the present invention.
Casting was also performed for the cases of 40 mm and 150 mm.

Further, as a comparative example, the immersion depth H is set to 5 even in the three types of immersion nozzles in which the discharge hole angle θ of the immersion nozzle is upward 10 °, 0 ° (horizontal), and downward 5 °.
The casting was carried out in the range of 0 mm to 175 mm, and the other conditions were the same as in the above example.

The surface of the obtained slab was examined by a penetrant inspection method to measure the length and number distribution of vertical cracks. After the slab surface was scarfed by 2 mm, the scarf surface was examined by a penetrant inspection method. The size and number distribution of norokami were measured. Then, the length and the number distribution of the vertical cracks, and the size and the number distribution of the norokami were statistically processed by exponential function approximation, and the vertical crack rating and the norokami score were determined to evaluate the surface condition. In addition, the lower the occurrence rate of the defect, the lower the score of both the norokami and the vertical crack score, and the score 0 indicates that no defect occurs.

FIGS. 2, 3, 4, 5, 6, and 7
Means that the discharge hole angle θ of the immersion nozzle is
Degrees, 5 degrees upward, 2 degrees upward, 10 degrees upward, 0 degrees (horizontal), and 5 degrees downward,
It is shown that the norokami score and the vertical crack score are divided for each immersion depth H.

As shown in FIG. 2, FIG. 3, and FIG. 4, when the discharge hole angle θ is within the range of the present invention and the immersion depth H is in the range of 80 to 130 mm, both norogami and vertical cracking occur. A slab free of surface defects and free of surface defects could be obtained.
However, even when the discharge hole angle θ is within the range of the present invention, when the immersion depth H is less than 80 mm, mainly slime occurs,
When the immersion depth H exceeded 130 mm, longitudinal cracks mainly occurred.

As shown in FIGS. 5, 6, and 7,
If the discharge hole angle θ is out of the range of the present invention, the immersion depth H
When the immersion depth H was made shallow, slivers were generated, and when the immersion depth H was made shallow, nookami and vertical cracks could not be prevented at any immersion depth H.

[0030]

According to the present invention, the discharge hole angle θ of the immersion nozzle is
Since the high-Ti-content steel is continuously cast while limiting the immersion depth H to the optimum range, a sound slab having no lozenges and no vertical cracks can be obtained and can be rolled without care. As a result, the yield is improved and energy saving is achieved, so that the industrial effect is extremely high.

[Brief description of the drawings]

FIG. 1 is a schematic view of a front section of a mold portion of a continuous casting machine having a rectangular cast slab to which the present invention is applied.

FIG. 2 is a diagram showing a norokami score and a vertical crack score when the discharge hole angle θ is 8 degrees upward, divided for each immersion depth H.

FIG. 3 is a diagram showing a norokami score and a vertical crack score when the discharge hole angle θ is 5 degrees upward, divided for each immersion depth H.

FIG. 4 is a diagram showing a norokami score and a vertical crack score at an ejection hole angle θ of 2 degrees upward divided for each immersion depth H.

FIG. 5 is a diagram showing a norokami score and a vertical crack score at a discharge hole angle θ of 10 degrees upward, divided for each immersion depth H.

FIG. 6 is a diagram showing a norokami score and a vertical crack score at a discharge hole angle θ of 0 degree (horizontal), divided for each immersion depth H.

FIG. 7 is a diagram showing a norokami score and a vertical crack score when the discharge hole angle θ is 5 degrees downward, divided for each immersion depth H.

[Description of Signs] 1 Immersion nozzle 2 Discharge hole 3 Mold 4 Mold long side 5 Mold short side 6 Molten steel 7 Meniscus 8 Mold powder 9 Discharge flow 10 Solidified shell 11 Tundish

Claims (1)

[Claims] 1. A continuous casting method for a high Ti content steel containing 0.1 to 1.0 wt% of Ti, wherein a discharge hole angle θ of an immersion nozzle is set within a range of 2 to 8 degrees upward and an immersion nozzle is provided. Wherein the immersion depth H is in the range of 80 to 130 mm.